The present invention relates generally to a liquid level indicator such as a fuel level indicator used in airplanes, boats and automobiles, and more particularly to a liquid level transmitter with magnetically coupled rotors that does not introduce electrical energy into the fuel tank.
Without limiting the scope of the invention, its background is described in connection to fuel tanks, more specifically aircraft fuel tanks.
Liquid level measurement in aircraft, automobiles, boats and other vehicles has historically been measured by either of two methods, float or capacitance probe. Each of the two methods in common use are discussed below. In each of these techniques the fuel tank and its contents are subject to electrical energy in the measuring technique. Recently there have been serious safety concerns due to unexplained aircraft losses which may have been caused by a spark from the electrical equipment inside the fuel tank. Also, systems are described that have been proposed but either have not been implemented, have been implemented with poor results or have been implemented on a test basis only.
Float indicators were used on early aircraft since the aircraft lacked an electrical system. The initial non-electrical float system was used on various aircraft, the most famous being the Piper J-3 xe2x80x9cCub,xe2x80x9d which used a cork with a wire imbedded in it that extended into the view of the pilot. The fuel tank in a Cub is directly in front of the windshield and the tank has a cap with a hole that allows the wire to move up and down. The wire has a bend near the top end so that the end of the wire cannot fall into the fuel tank. Additionally, the bent wire is more visible to the pilot than a straight wire. The float. and wire indicator operate on a simple principle: lots of wire showing, lots of gas; no wire showing, no gas.
With the advent of aircraft and cars with electrical systems the float is connected to the arm of a variable resistor whose electrical leads are brought through the wall of the tank. The fuel quantity gauge is connected to the resistor leads and to the vehicle""s electrical system. Typically, when the float is on the bottom of the tank, the resistance sensed is low and when the float is high, the resistance sensed is high, on the order of 30 Ohms. This causes the needle on the fuel quantity gauge to deflect as the float height varies thus indicating the quantity of fuel in the tank. For odd shaped tanks, particularly a flat tank in a wing with dihedral, the resistance floats may be connected-in series to cover this longer sloped tank. Techniques exist to calibrate the readings on the gauge, which may be either digital, indicated by a discrete number, or analog, indicated by a needle position.
A resistance float device is used on most, if not all, automobiles, all piston engine aircraft, and some turbine aircraft. This system has been given very poor reviews over the years. If the resistance float is poorly designed and constructed, the gauge is poorly designed and constructed, the gauge poorly marked, the damping not suitable for aircraft use, or if the system is poorly installed and calibrated, the criticism is deserved. This system also introduces electrical energy into the fuel tank.
Capacitance probes are another method used to measure liquid level, especially in fuel tanks. This system uses two concentric tubes arranged in the form of a probe inserted into the top of the fuel tank. Since the dielectric constant of fuel is radically different from that of air, a measure of the height of the fuel level can be made by measuring the capacitance between the two tubes. This system is used on most turbine aircraft and is convenient to install on deep tanks.
The spacing between the tubes can be used to provide a linear output in an odd shaped tank. These systems are generally expensive since the interconnecting wiring must be coaxial cable and some sort of a processor must be used to sum and linearize the probe outputs. Since the dielectric constant and density of turbine fuel varies with temperature, and since the pilot of a turbine installation would like to know the mass of the fuel in the tank rather than the volume, the capacitance probe must have a compensator probe built in to compensate for the dielectric constant to provide an output representing mass, . . . i.e. pounds of fuel. This system also introduces electrical energy into the fuel tank.
In the early days of aviation, an air pressure technique was used to measure the quantity of fuel in a tank. Basically, this method consisted of an air pressure gauge and an air pump connected to an air outlet at the bottom of the fuel tank. When the pump pressurized the gauge and the plumbing connected to the air outlet, the reading on the air pressure gauge increased until the air pressure exceeded the head pressure of the fuel tank and air began to bleed from the air outlet stabilizing the pressure in the system. At this point the reading on the air pressure gauge is equal to the head pressure of the fuel and is indicative of the mass of fuel in the tank. The pressure pump had to be properly designed to avoid excessive flow and thus an erroneous reading.
There has been an attempt to bring this method into the modern age. Systems have been built that used a differential pressure transducer measuring the head pressure in excess of atmospheric pressure. This output is then compensated for acceleration of the fuel mass in a dynamic environment, that is, at a varying distance from the surface of the earth to produce a xe2x80x9cstablexe2x80x9d indication of the mass of fuel in the tank. This technique suffers from the problem of attempting to measure a small reading which is the difference between two relatively large numbers. In this situation the errors generally overwhelm the attempted measurement.
There has even been an attempt to measure potable water by weighing the container, subtracting the bare weight of the container, and correcting for acceleration. This is arguably a more accurate method since the weight of the contents is much smaller than the weight of the container, but the problem with this method is that the container must be isolated from the structure while it is on the xe2x80x9cscales.xe2x80x9d This method works for potable water since the container is generally removed from the vehicle for filling, but is not a practical solution for fuel tanks. These techniques do not introduce electrical energy into the fuel tank.
There are a number of schemes based upon internal reflection of light in a polycarbonate rod in the tank. This optical method is most often used for low fuel and low oil measurements and is in wide use in the aviation market. Another common application is the xe2x80x9cmagic eyexe2x80x9d on certain automotive batteries to indicate the level of the electrolyte in the battery. These systems function as a xe2x80x9cYes/Noxe2x80x9d reading. When the liquid covers the end of the probe the magnitude of the reflected light is radically different than when the end of the probe is in air. In order to use this technique to measure a continuously changing liquid level, the sensor must either have xe2x80x9cXxe2x80x9d discrete sensors where xe2x80x9cXxe2x80x9d is the resolution desired or the probe must be designed to internally reflect a varying quantity of light with a varying level of liquid. The multi probe method is not practical in a moving aircraft, while the variable reflectivity method has calibration and long term stability problems. In general, these techniques do not introduce electrical energy into the fuel tank.
Magnetic methods of liquid level measurement utilizing a Hall Effect semiconductor device are discussed in the Honeywell Solid State Sensors Catalog. Determining the height of a float is one method of measuring the level of liquid in a tank. A linear output Hall Effect transducer in placed outside of the tank while a magnet is placed inside the non-ferrous metal tank, and moved by the motion of a float arm. As the liquid level moves up or down, the magnet moves relative to the transducer, causing a change in transducer output voltage. This system allows liquid level measurement without any electrical connections inside the tank. This method does, however, require some electronic interface to allow the output to be used with a gauge. Additionally, there are linearity and temperature effects that must be either compensated or suppressed. A set of transducers using this technique have been built and flown with promising results; however, the output must be linearized and additional work must be performed to insure temperature stability.
While the above referenced techniques are useful in measuring liquid level in some circumstances, they do not provide a reliable method of measuring liquid level without introducing electrical energy into the tank. What is needed is a device for measuring liquid level without introducing electrical energy into the tank. A device capable of reliably and inexpensively measuring liquid levels without introducing electrical energy into the tank would provide numerous advantages.
The present invention provides a fuel transmitter utilizing a magnetic drive with two magnetic rotors placed on either side of a non-ferrous plate which is used to cover an opening in either the side wall or top or bottom of the tank. Placement depends upon the specific application and configuration. The rotors, while on opposite sides of the plate, are arranged on axles each located on a common axis. Thus, when one rotor is turned, the other rotor follows the rotation due to the magnetic coupling between the two rotors. The torque available is a function of the offset of the poles (rotor diameter), magnetic strength (Gauss Level), and the distance between the rotors (Gap distance). Sufficient torque must be available to ensure close tracking of the following rotor and its readout device with the position of the driving rotor.
In one embodiment, a first rotor, the drive rotor, is attached to a float assembly. As the float assembly moves with changing liquid levels, the first rotor turns. The movement of the first rotor is followed by a second rotor, the output rotor. Although the first and second rotors are not physically connected, the rotors are bound together with a magnetic coupler. The magnetic coupler keeps the first and second rotors aligned and joined so that the second rotor will move the same angular displacement as the first rotor in response to the position of the float assembly. The second rotor turns a means for producing a liquid level quantity signal. For example, a variable resister may be used to produce a liquid level quantity signal.
In another embodiment, a system is disclosed for determining liquid levels in a tank. The system has a float connected by a float arm to a dual rotor liquid level transmitter. As the float moves within the tank, the liquid transmitter senses the change and transfers the movement to a gauge by a volume signal.
In another embodiment, the system further comprises a temperature probe and a logic device for converting the volume signal into a mass signal accounting for the temperature of the liquid. A mass readout is desirable for some liquids such as fuels used in turbine engines. For example, Jet A, JP4, JP-5, and JP-8 are normally measured in pounds mass instead of gallons in an aircraft.
In another embodiment, a method is disclosed for determining the quantity of fuel in a tank, without introducing electrical energy into the tank, by translating the position of a float from a first liquid level within the tank, sensing the rotational displacement of a first rotor magnetically coupled to a second rotor to rotate together about a common axis as the liquid level changes within the tank. The method comprises steps of adding a measured first quantity of liquid for a first liquid level to a tank for a first float position. Next, the signal produced by the first float position is detected and a measured second quantity of liquid to the tank for a second float position corresponding to a second liquid level is added. Finally, the signal produced by the second float position is detected.
A technical advantage of the present invention is the provision of a liquid level fuel transmitter that does not introduce electrical energy into the tank.
Another technical advantage of the present invention is the reduced complexity and cost of the fuel transmitter of the present invention as compared to prior art devices.